The present invention relates generally to a communication apparatus and a communication method for broadcasting systems or communication systems for receiving application data signals such as audio, video, data and the like signals for transmission to receiver terminals. In particular, the invention is concerned with communication control apparatus and method for such system in which concealment of data communication is required. More particularly, the present invention is directed to personal computer communication system, video distribution system for personal computers, cable television (CATV) system, terrestrial broadcasting system, satellite broadcast system, satellite communication system and the like, and is concerned with a communication apparatus and a communication method therefor which are capable of establishing synchronism without fail between changeover of scramble keys used for encrypting or scrambling the data for transmission (hereinafter also referred to as application data) in order to secure concealment of such data and multiplexing thereof with time-division frames which contains information of the scramble keys.
Heretofore, as the method for switching or changing the scramble keys and time-division frames for ECM data (abbreviation of Entitlement Control Message data), there has been known a method mentioned below.
A multiplexer or multiplexing unit for multiplexing application data such as audio and/or video data as well as data in intrinsic sense is provided with an input port dedicated for the ECM data in addition to those for the application data mentioned above. In that case, the ECM data is imparted with higher level of priority for multiplexing than the application data inputted by way of the other input ports. The multiplexing unit is so designed as to operate such that the time taken for the multiplex processing of the data inputted through respective input ports can remain constant. Alternatively, a maximum time taken for the processing by the multiplexer as a whole is set as a preset multiplexing time. Upon every lapse of constant or preset time interval, the scramble key is switched or changed over. The scramble key as changed is then contained in the ECM data and inputted to the multiplexing unit to be thereby multiplexed and transferred to a scrambler, which responds thereto by scrambling the application data with the scramble key as received. In the conventional communication apparatus known heretofore, synchronism is established between the switching or changeover of the scramble key and that of the ECM data in the manner described above.
As is apparent from the above, the timing for multiplexing the ECM data into a data stream is determined by the multiplexing unit. Additionally, the timing for changing the scramble key is effected at a predetermined constant time interval within the scrambler. Under the circumstances, there may arise such situation that if the processing has not been completed within the predetermined processing time preset in the multiplexer for some reason, the ECM data containing the information of old scramble key, so to say, will be outputted from the multiplexing unit to be transmitted as the valid ECM data. In that case, it becomes impossible to restore the application data from the scrambled state, i.e., to descramble the application data at the side of the receiver apparatus. Obviously, influence of such event will propagate through the whole system, giving rise to a serious problem that the service is interrupted temporarily at the side of the subscribers or receivers.
Besides, when the conventional scramble key change procedure is adopted, it is impossible to manufacture the units such as the multiplexer and the scrambler in the specifications independent of each other. For this reason, when a transmission line or channel which need not ensure concealment of data for transmission is newly imposed with requirement for the concealment, then a multiplexer of other specifications will be required, to a disadvantage. Needless to say, such multiplexing unit will be expensive because of higher performance and rigor specifications as demanded.